1. Field of the Invention
The present invention relates, in general, to an electroluminescent polymer and an electroluminescent device using the same. More specifically, the present invention pertains to an electroluminescent polymer comprising a main chain of phenylene vinylene and a side chain of carbazole and an aliphatic alkyl or alkoxy group, and an electroluminescent device fabricated using the electroluminescent polymer.
2. Description of the Prior Art
With great advances in the field of electronics made possible by the use of silicon as a semiconductor material, people have enjoyed various modern conveniences. Recently, the rapid growth in the optical communication and multi-media fields has accelerated the development of information communities. Consequently, optoelectronic devices that convert light energy into electric energy and vice versa are very important in the electronic information industry nowadays. Such semiconductive optoelectronic devices can be classified as electroluminescent devices, semiconductive laser devices, light-receiving type devices, etc.
A flat panel display generally means a device that solves inherent difficulties in picture processing by the conventional CRT (cathode ray tube) mode, and a device that expresses images of at least the same quality as that of the CRT mode. The earlier display was mostly used for wall-mounted monitors. It has been applied to computer monitors, notebook PC""s, PDA terminals, etc., for modern information technology and multi-media devices. Most of the display means are of the light-receiving type, whereas an electroluminescent (EL) display is of the light-emitting type. The EL display has advantages including a fast response, obviation of a backlighting effect, and excellent luminance, so that many applications therefor are under study. Such EL devices are fabricated with inorganic semiconductors using GaN, ZnS and SiC, and thus are used as practical displays. In the case of EL devices prepared from an inorganic material, as the driving voltage is over 200 V and the manufacture of the EL device is carried out by means of a vacuum deposition process, a large size device cannot be prepared and the cost of preparation is very high.
However, Eastman Kodak Company presented a device prepared with a pigment having a xcfx80-electron conjugated structure, called xe2x80x9calumina-quinone, Alq.sub.3xe2x80x9d in 1987, and thereafter research into EL devices using organic material has been active. In the case of using an organic material having low molecular weight, a synthesis procedure is simple, it is easy to synthesize materials in various forms, and color tuning is possible. However, such materials have disadvantages including low mechanical strength and low crystallization upon heat exposure. To overcome the above disadvantages, research is being conducted to develop an organic EL device having a polymeric structure. In the xcfx80-electron conjugated polymer, the energy level is split into a conduction band and a valence band by the overlap of xcfx80-electron wave functions which exists in the polymer main chain, various semiconductive properties of polymer are decided by a band gap energy which corresponds for the energy difference between the conduction band and the valence band, and a processing of full color is possible. Such a polymer is referred to as a xe2x80x9cxcfx80-electron conjugated polymerxe2x80x9d. Research to develop electroluminescent polymers has been carried out in Cambridge University, England, 1990, producing poly(p-phenylene vinylene) (hereinafter, xe2x80x9cPPVxe2x80x9d), a polymer having conjugated double bonds, and thereafter attempts have been made to apply xcfx80-electron conjugated polymer to an electroluminescent display.
Meanwhile, luminous materials of high purity and optimized structure are required to increase the electroluminescent effects of EL devices. In electroluminescent devices, holes and electrons within the polymer used as a light emitting layer may be efficiently bound, and thus luminous efficiency is enhanced. That is to say, it is important that the injected charge amounts are kept in equilibrium for high luminescence, thereby balancing the charge density between holes and electrons. However, in PPV-based derivatives having p-type semiconductive properties, since the transport rate of holes is much faster than that of electrons, this difference reduces the luminous efficiency. So, in the EL device structure, a novel membrane layer is introduced for blocking the transportability of holes, or any materials having good electron transportability for increasing electron-injection may be used.
It is a feature of the present invention to provide an electroluminescent polymer comprising PPV as a main chain and carbazole and an aliphatic alkyl or alkoxy group as a side chain introduced to the PPV chain, which is advantageous in terms of excellent luminous efficiency, high solubility in organic solvents, and superior membrane properties, by controlling the transportability of holes and balancing the charge density between holes and electrons.
It is another feature of the present invention to provide a copolymer of a monomer of the above electroluminescent polymer and a PPV-based monomer.
It is further feature of the present invention to provide a composition comprising a mixture of the above electroluminescent polymer and a PPV-based polymer.
It is a still further feature of the present invention to provide an electroluminescent device using the above electroluminescent polymer.
In accordance with one aspect of the present invention, there is provided an electroluminescent polymer, represented by the following formula (1): 
wherein X1 to X5 are independently a hydrogen atom, a linear alkyl or alkoxy group having 1 to 40 carbon atoms, a branched alkyl or alkoxy group having 3 to 40 carbon atoms, a cyclic alkyl group having 5 to 40 carbon atoms, a silyl group, or an aromatic group having 6 to 14 carbon atoms which is unsubstituted or substituted with at least one selected from the group consisting of an alkoxy group having 1 to 40 carbon atoms and an amine group. Examples of the cyclic aliphatic groups include cyclohexyl group, admantyl group, etc. Examples of the silyl group include trimethylsilyl group, dimethyloctylsilyl group, etc. Examples of the aromatic group include phenyl group, naphthyl group, etc.
In accordance with another aspect of the present invention, there is provided an electroluminescent polymer, comprising a monomer of the above electroluminescent polymer and a PPV-based monomer, represented by the following formula (3): 
wherein X1 to X9 are respectively a hydrogen atom, a linear alkyl or alkoxy group having 1 to 40 carbon atoms, a branched alkyl or alkoxy group having 3 to 40 carbon atoms, a cyclic alkyl group having 5 to 40 carbon atoms, a silyl group substituted with at least one alkyl group having 1 to 40 carbon atoms, or an aromatic group having 6 to 14 carbon atoms which is unsubstituted or substituted with at least one selected from the group consisting of an alkoxy group having 1 to 40 carbon atoms and an amine group, and a and b are numbers such that 0.1xe2x89xa6a/(a+b)xe2x89xa60.9, and wherein at least one of the X substituents is a group other than a hydrogen atom. Examples of the cyclic aliphatic group include cyclohexyl group, admantyl group, etc. Examples of the silyl group include trimethylsilyl group, dimethyloctylsilyl group, etc. Examples of the aromatic group include phenyl group, naphthyl group, etc.
In accordance with a further aspect of the present invention, there is provided an electroluminescent polymer composition wherein the above electroluminescent polymer and a PPV-based polymer are mixed in the weight ratio of about 1:99 to about 99:1, more preferably between about 5:95 to about 95:5.
In accordance with a still further aspect of the present invention, there is provided an electroluminescent device, comprising a structure selected from the group consisting of an anode/light emitting layer/cathode, an anode/buffer layer/light emitting layer/cathode, an anode/buffer layer/hole transport layer/light emitting layer/cathode, an anode/buffer layer/hole transport layer/light emitting layer/electron transport layer/cathode, and an anode/buffer layer/hole transport layer/light emitting layer/hole blocking layer/cathode, wherein the above electroluminescent polymer or the above electroluminescent polymer composition is contained in the light emitting layer.
Methods of producing the inventive polymers are also provided.
These and other features and aspects of the present invention will be readily apparent to those of ordinary skill in the art upon review of the detailed description that follows.